Ch 5 Hill Road

Chapter 4

Hill Roads

7/30/2012 1Ch5 Hill Road Er.Sunil Khyaju

COURSE OUTLINE

– Introduction

– Special consideration in hill road design

• Alignment of hill road design: general consideration,

route location hills, gradient, design and types of hair

pin bends, types of hill road cross sectionspin bends, types of hill road cross sections

– Special structure in hill road

– Types retaining structures, river training structures, land

slide stabilization structures and gully control structures


INTRODUCTION

• According to Nepal Road Standard 2045,

terrain are classified as percent cross-slope as;

• A hill road is defined as the one, which passes

through a cross-slope of 25% or more i.e.

mountainous or steep terrain.


Design and construction problems of

hill roads

Design and construction of a hill road is more complicated than

plain area. It is because of several factors associated with such

area some of them are:

• A hilly or mountainous area is characterized by a highly

broken relief with widely differing elevations, steep slopes.

This may cause unnecessary increase in length of the road.This may cause unnecessary increase in length of the road.

• The formation of rock differs in a wide range. The geological

condition varies from spot to spot. This will make difficult to

assess the foundation and find the suitable one for the road

embankment and other road structures.


• Cross slope highly stable before construction may turn into

more unstable after construction.

• Variation in hydrological condition from place to place is

difficult to perceive and may be easily overlooked in design

phase which causes various damages of roads after

construction.

• A highly broken relief is the main reason requiring the

installation of various types of special road structure such as

stepped culverts, aqueduct, retaining structures etc.

• Presence of high cross slope needs a careful arrangement of

erosion protection works.


• Due to presence of high cross slope surface water reaches

quickly to road side with high speed. In its course to road side

it may bring lot of debris (land slide may be followed) which

has higher damage strength.

• Variation in climate condition should be considered in design

and construction period, such as:

– Temperature decreases as the height increases.

– Pressure (atmospheric) decreases as the altitude – Pressure (atmospheric) decreases as the altitude

increases.

– Precipitation increases as the height increase

– Velocity of wind varies from place to place depending

upon the location of valley.

• Design of hair pin bends to gain height

• Frequent blasting are required due to the presence of hard

rocks etc.7/30/2012 Ch5 Hill Road Er.Sunil Khyaju 6

Alignment Selection in Hill Road The selection of alignment in hilly region is a complex problem.

The main target of designer is to search the shortest possible

short route. Following points are considered in alignment

selection.

a. General Consideration:

• A hilly area is characterized by a highly broken relief with

widely differencing elevations, steep slopes, repeated turns

and bends.

• When designing hill roads the route is located along valley,

hill sides and mountain pass.

• Most of the work has to be carried out in rock using

explosives and retaining walls.

7/30/2012 Ch5 Hill Road Er.Sunil Khyaju 7

• Unfavorable geological conditions such as landslides may be

encountered and special structures have to be provided to

establish the stability of road.

• In locating the alignment of hill road and structures following

points are considered.

I.Temperature:

–The temperature of air varies inversely with altitude. The –The temperature of air varies inversely with altitude. The

temperature drop being about 0.5°C per 100 m of rise.

–Similarly the amount of heat received by hill slopes varies

with their orientation in relation to the exposure to sun.

– On slopes facing south and south west snow disappears

rapidly and rain water evaporates quickly.


II. Rainfall:

• The amount of rainfall in a hilly region is inversely

proportional to the altitude.

• The maximum rainfall is in the zone of intensive cloud

formation (1500-2500m) above the sea level after which it

decreases substantially.

• Mountain range slopes with face winds coming in from the

sea receive more rainfall than other side of the range. sea receive more rainfall than other side of the range.

III. Atmospheric pressure and winds

• The variation of atmospheric pressure and wind speed may

cause snowfall, snow drift (mass of snow thrown by wind)

and avalanches (mass of snow in mountains).

• So it is very important for designer to be familiar with the

local climatic condition.


IV. Geological condition:

• The degree of stability of hill slope depends upon the type of

rock, the degree of strata inclination, hardness of rock and

presence of ground rock.

• Sedimentary rocks have tendency to slip under the influence

of force parallel to the layer.

• The instability of hill road may be due to ground water, • The instability of hill road may be due to ground water,

landslide and unstable folds.


b) Route location:

• Hill routes tend to follow tortuous routes with large number of curves to bypass obstructions, to cross water courses and to develop the route for negotiating elevation differences.

• The approach to the location of hill road alignment • The approach to the location of hill road alignment varies for the section along the valley bottom (river route) and along the mountain pass (ridge route).

• There are both advantages and disadvantages of both locations.


i. River route:


• The location of a route along the river valley is known as river route. River route is generally used in hill road due to comparatively gentle gradient.

• The advantages of river route are low vehicle operation cost, availability of water and other construction material in the vicinity.

• However, a river route may involve numerous horizontal curves, construction of large bridges over tributaries and on However, a river route may involve numerous horizontal curves, construction of large bridges over tributaries and on the stretches along steeply sloping hill sides, which in some places may be unstable.

• It may also be necessary to construct special structures and massive river training and protection structures on the valley sides to safeguard the road against washout, toe cutting etc.


ii. Ridge Route:


• A ridge route is characterized by very steep gradient, numerous sharp curves including hair pin bends and expensive rock work.

• The road usually follows the top section of the hill system and crosses successively mountain pass (location in the range of mountains that is lower than the surrounding peaks).

• The ridge route climb continuously up from the valley floor till • The ridge route climb continuously up from the valley floor till it reaches a mountain pass and then descends down to catch another hill system.

• Geologically stable and comparatively mild slope sections are selected for the development of the route. The route is traced in the map by following more or less the line of equal gradient.


Design and types of hair pin bends

• To attain height at a particular location

without attaining substantial covering of

horizontal distance , in such cases hair-pin

bend is provided.

• When developing a route in hilly area, it is • When developing a route in hilly area, it is

frequently necessary to insert sharp turning

angle, within which it is very difficult and

sometimes impossible to layout curves

following normal geometric design.


Design criteria for hair pin bends

The straight length between two successive hair pin bends should be minimum of 60 m excluding the length of circular and transition curve.

• Minimum design speed = 20 kmph

• Minimum radius of inner curve = 14 m

• Minimum length of transition curve =15 m

• Super elevation in circular portion of the curve = 1 in 10• Super elevation in circular portion of the curve = 1 in 10

• Minimum width of carriage way at the apex of curves should

be 11.5 m and 9 m for two lanes and one lane respectively.

• The maximum and minimum gradients should be 1 in 40 and 1 in 200 respectively at the curve.

• For good visibility the island portion should be cleared of all the trees.



Types




Typical cross sections of hill roads

a) Cut and fill



b) Benching cutting


c) Box cutting


d) Embankment

with retaining

walls


e) Semi bridge


f) Semi tunnel


g) Platform


Main Structures in Hill Roads

On the basis of function of structures, these are classified into three groups:

a) Retaining structures a) Retaining structures

b) Drainage structures

c) Slope protection structures


Retaining structure• A retaining structure is usually a wall constructed for the

purpose of retaining a vertical or nearly vertical earth bank

which in turn support vertical load.

• It is most important structure in hill road construction.

• It provides adequate stability to the slope.






b) Drainage structures: • Drainage is one of the main problems during

construction as well as in operation of roads. Some of the features of hill road drainage system are:

1. Drainage of water from hill slope:

2. Road side surface drainage:

3. Sub-surface drainage:

4. Cross-Drainage:


1. Drainage of water from hill slope:


2.Road side surface drainage:


c. Cross-Drainage:



c) Slope protection structures


Passing lane in hill roads • The construction of hill road is not only costly but

also tedious work due to abrupt change in level,

geological conditions and limited funds. Sometimes it

is very difficult to cut the hard rock while sometimes

it is very tedious to make stable flow ground. So the

width of hill road is not uniform throughout. Width

of hill road is fixed based on the geological of hill road is fixed based on the geological

conditions, traffic volume, method of construction

and availability of fund. At certain interval some

extra space is provided to pass the traffic coming

from opposite direction or to overtake the slow

moving vehicle. This type of arrangement is known

as passing lane in hill road. 7/30/2012 Ch5 Hill Road Er.Sunil Khyaju 42

Ch 5 Hill Road

Documents

hill road types

road structures

3ch5 hill road

5ch5 hill road

4ch5 hill road

road embankment

nepal road standard

sunil khyaju cross slope